Eddy current testing is a well known process in which a small coil of wire is used to produce an electromagnetic field proximate a conductive material. The electromagnetic field interacts with the conductive material to induce an eddy current, which in turn produces a secondary electromagnetic field of opposite polarity to the primary field. The coil is connected to instruments that monitor the effect of the interacting fields on the coil, in addition to providing the current that produces the primary electromagnetic field. In the absence of defects in the conductive material, the eddy current signals are relatively small in amplitude, and the monitor interprets the electrical characteristics of the coil accordingly. On the other hand, a defect or discontinuity in the conductive material interrupts the paths of the eddy currents, causing a measurable increase in the amplitude of the eddy current signals, and the monitor interprets the change in the electrical characteristics of the coil accordingly. The increase in signal amplitude is a function of the size of the discontinuity in the conductive material.
Eddy current testing has a wide range of applications, including detection of defects in aircraft parts and other types of high speed turbomachinery having bolt holes formed therein. In this particular application, one of several probes is selected according to the diameter of the bolt hole, and the chosen probe is connected to conventional eddy current testing equipment. The system is calibrated for the chosen probe, and then the probe is inserted into the bolt hole so that an eddy current coil on the probe is proximate the side of the bolt hole.
One type of eddy current probe, commonly known as a non-contacting probe, has a fixed diameter approximately 10 mils less than the diameter of the bolt hole to be inspected. The probe is rotated at approximately 1500 rpm as it is inserted into the bolt hole. Since the quality of the inspection is a function of the proximity of the eddy current coil to the wall of the bolt hole, a separate probe is required for each variation in bolt hole diameter that exceeds 5 mils.
Another type of eddy current probe, commonly known as a contact probe, has a diameter no smaller than 20 mils less than the diameter of the bolt hole to be inspected, and no greater than 5 mils more than the diameter of the bolt hole to be inspected. This type of probe also requires calibration of the operating system each time a different probe is connected thereto. The contact probe is rotated at approximately 500 rpm as it is inserted into the bolt hole. In order to position the coil as close as possible to the wall of the bolt hole, this type of probe is biased into contact with the bolt hole wall. To accomplish this biasing, the conventional contact probe has an eddy current coil mounted on one of a pair of flexible fingers. If the effective diameter of the relaxed fingers is slightly greater than the diameter of the bolt hole, then the probe is simply wedged into the bolt hole. If the effective diameter of the relaxed fingers is less than the diameter of the bolt hole, then a wedge is inserted between the distal ends of the fingers to force them outward into contact with the wall of the bolt hole. Although somewhat tedious, this wedging technique is suitable for varying the effective diameter by as much as 25 mils.
Both of the probes described above suffer from their relative lack of diameter adjustability, because bolt hole diameters vary significantly. Changing probes and recalibrating the system for each newly substituted probe is time consuming. Moreover, a complete bolt hole inspection set must include a series of eddy current probes ranging in effective size from the diameter of the smallest encountered bolt hole to the diameter of the largest encountered bolt hole. Even with the adjustability afforded by wedging the conventional contact probe, seventeen such probes are required to inspect bolt holes ranging in diameter from 0.200 inches to 0.600 inches. The need to stock so many functionally identical tools is expensive, as well as inconvenient. Accordingly, a need exists for an eddy current probe that is capable of inspecting bolt holes ranging significantly more than 25 mils in diameter.
Others have recognized this need for an adjustable diameter bolt hole probe, as evidenced by U.S. Pat. Nos. 4,262,425 and 5,136,240, both of which disclose self-adjusting eddy current probes. Although on paper, each of these patented devices may appear to solve the need for adjustability, in practice, neither of these devices has been particularly well received in the industry. The devices provide a greater range of adjustability but not without shortcomings. For example, both patented devices, as well as the "wedgeable" contact probe described above, have sidewalls that flex in an arc toward the wall of the bolt hole to be tested, thereby varying the angle of the eddy current coil relative to the bolt hole wall.
Another problem arises from the fact that each of the self-adjusting devices has an outwardly biased expansion mechanism that provides an expansion bias as a function of the magnitude of expansion. In other words, the eddy current coil is pressured more against the wall of a bolt hole having a relatively small diameter, and less against the wall of a bolt hole having a relatively larger diameter. Accordingly, calibration may still be required between tests of different sized bolt holes, even though the same probe may be used in both tests. Moreover, it is unclear to what extent either of these patented devices eliminates the need for multiple probes. In this regard, it is believed that the device disclosed in U.S. Pat. No. 5,136,240 might expand from a minimum diameter of 0.250 inches to a maximum diameter of 0.300 inches, for example.
Another problem with the outwardly biased expansion mechanisms is that the probe could become stuck if it were accidentally inserted all the way through a bolt hole, because the sidewalls would be biased beyond the confines of the bolt hole. In view of the foregoing, the need remains for an effective bolt hole probe having an adjustable diameter range of several tenths of an inch.